Ophthalmic lens is widely used and the high demand has created a need to produce very high quality lenses both standard and cosmetic lenses, in large quantities. It is a generally a known fact that lenses produced in an automated manufacturing line is more reliable than those manufactured by a manual system of production which suffers from unpredictable problems. It is also a widely accepted fact that inspection systems are an essential part of an automated manufacturing line to inspect the lenses and maintain a consistent and high quality inspection process to deliver premium quality product to customers. Regular tweaking of parameters and generating configuration files that contain inspection characteristics for various ophthalmic lens models enables flexible adaption of the inspection system for different types of lens. Ophthalmic lenses are intended for use in the human eye not just to correct the vision but also to enhance the cosmetic look of the eye by printing designs on the ophthalmic lens. Therefore great care must be taken to ensure that they are defect free. These ophthalmic lenses are produced in very high volumes in an automated manufacturing line. In order to ensure that each one of the lenses is manufactured as per strict quality control standards, it is essential that the lenses are inspected just before packaging, using automated inspection methods.
Before packaging, the ophthalmic lenses are placed in lens holders, which are transparent. Each holder holds one lens, which is generally immersed in a liquid solution. The wet lens inside the holder is inspected as the lens carrier moves along the conveyor in an automated manufacturing line. In order to increase the throughput of the manufacturing line, it is essential that the lenses are inspected as quickly as possible.
The holder used should preferably be made of clear glass with no coating on its bottom surface. Where the bottom surface of the holder is coated, it can result in images with uneven background.
The commonly used techniques for inspection of wet ophthalmic lenses are bright-field imaging, dark-field imaging and images captured using illumination in the Infra Red region. There are different types of illumination that use halogen lamps, xenon lamps, Lighting using LEDs. Etc. By using different illumination, many different types of defects are enhanced, which makes the images captured easy to detect very fine types of defects thus minimizing false rejects.
Prior Art U.S. Pat. No. 6,765,661 discloses the use of combining bright-field imaging and dark-field imaging in order to provide a good quality of inspection of defects. However, as is evident from later patents from the same inventor (Prior Art U.S. Pat. Nos. 7,663,742 and 7,855,782), the use of just bright field imaging in combination with dark-field imaging, is not enough for comprehensively inspect all types defects in anophthalmic lens.
Prior Art U.S. Pat. Nos. 7,855,782 and 7,663,742 disclose the use of combining phase contrast imaging along with either bright-field imaging or dark-field imaging in order to inspect all types of defects like accuracy of size, surface defects, tears, peripheral ruptures, inclusions such as bubbles and foreign bodies, as well as small defects at the edges of the ophthalmic lens.
In the prior arts discussed above inventions, monochromatic illumination light source is used along with beam splitters and other optics to split the light into secondary beams and one of the secondary beams is used for phase contrast imaging while the other is used for either bright-field imaging or dark-field imaging. By sequentially switching the light source for either bright-field imaging or dark-field imaging along with phase contrast imaging, complete inspection of the ophthalmic lens is achieved. This is time consuming and susceptible to Parasitic effect between illumination and external light. Furthermore, the use of holder with a flat bottom surface makes it difficult to consistently locate the edge of object to be inspected, as they can move around the holder. The lack in depth of focus also affects the sharpness of the image at the some areas of the ophthalmic lens, which impacts the edge detection ability and subsequent inspection of defects.
There is therefore a need for capturing multiple high-resolution images of the ophthalmic lens using multiple cameras for inspection, under different illumination wavelengths, different polarization, and to be able to trigger the illumination and camera shutters at the same instance or different instances as the user chooses, without sacrificing the image quality. This is the objective of the present invention.